In a memory, e.g., a flash memory, sometimes a word line's voltage needs to be higher than a power supply voltage VDD in order to effectively fulfill certain functions, e.g., read, erase, and/or program (write) functions. In some architectures, a thick gate-dielectric Metal-Oxide-Semiconductor (MOS) device is used as a high voltage device in a word line boost circuit to boost the word line voltage higher than VDD supply. For example, a thick oxide MOS device is used because during an erase operation, the MOS device is exposed to a high voltage stress (e.g., more than 13 V in a flash memory cell).
However, due to a relatively smaller gate capacitance in the high voltage device, a large number of high voltage devices are required to pump sufficient charge carriers into the word line's voltage supply and in turn to boost the word line voltage to a level higher than VDD in a reasonable time period. But a large number of high voltage devices consume a large chip area.